I have gotten better through the years, but I've spent much of my life as a doormat. Saying no was a nightmare for me, and in fact I found it so difficult to even hint at constructive criticism, I would avoid it at all costs. I've since learned to temper critical remarks with humor, so now I am better able to get my point across without being offensive, but it took me a long time to get to this place.
When I was in my mid 20's, I went through a phase where I was doing lots of volunteer work. I noticed that, consistently, the supervisors would bring up the fact that I was laid back. Even those who had only met me a few hours prior would mention that I seemed agreeable and easy to work with. I took this as a compliment at first, but then I had a conversation with my best friend about it.

"The only reason these people are telling you that you're so laid back is so that they can get you to do whatever they want. If they give you the label of the "laid-back" employee, you'll feel too guilty to ever say no to anything or to speak up when you're mad. They're totally manipulating you-screw them!"

She had a point. The more I was told how relaxed and passive I was, the more loathe I became to live outside of that label. If I every expressed irritation or anger, or even strong feelings about a situation, everyone around me was taken aback. This led to me constantly biting my tongue, and letting any resentment simmer to a long, slow boil. Great for everyone else around me, not so great for me.

Don't get me wrong; living up to an expectation of passivity isn't the worst thing in the world. But this is a great example of how we live up to the words and expectations of others. Get labeled as the passive one, or the angry one, or the poor confused soul, and you might just end up getting stuck in that role. Thoughts become action; they are the seeds from which reality springs. Self-perception is such a powerful tool; don't let anyone taint yours with their own misconceived notions of who you truly are.